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I have seen a few videos saying that there is a risk of getting an explosion because of hydrogen igniting if you attach the negative cable to the negative terminal on the car with the dead battery.

And they say that you should find a good ground for the negative terminal.

If I am not worried about the cars paint, can I just attach the negative cable to the bonnet or another part of the car's body? Thereby reducing the risk of damaging an electrical component.

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    When they say body they don't mean painted surface. You do not need to destroy your paint even if you do not care. There are many bare metal places under the hood to clip your cable to and these are the spots I am sure the video is referencing. – narkeleptk Feb 22 at 19:15
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Hydrogen is released when a typical car battery is charging. When you remove one jump lead after starting there could be a spark which ignites that hydrogen. It is unlikely to be when you attach the cables – the battery is not at that point being charged.

If you attach one cable to a bare metal point on the frame that is not right next to the battery, there are two benefits

  • the cable can be easier to clip than onto the battery

  • if there is a spark at that cable, it isn't near any hydrogen release

After starting the engine, you first remove the jump lead which is attached to the frame, away from where there might be hydrogen. Then removing the other cable can't make a spark, because now there is no circuit.

To be ultra sure, when you attach the cable to a frame point do it after you clip the other cable to the battery. You only make a spark when you complete, or break, an electrical circuit. So the cable clipped on the battery should be attached first, and removed last, because you don't want the spark to be right there at the battery.

But in the many times I have charged batteries and made jump starts, I never had a problem. I think the danger is minimal.

Two things to note

  • you can see which battery terminal that should be, because there is a thick cable running from one battery terminal to a nearby frame point

  • don't attach to the bodywork, only to a bare metal bracket inside the engine compartment

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    You thinking the danger of batteries exploding is minimal - that is bad advice. I have seen the damage from a battery exploding due to a spark during its connection. That caused 6 cars to need respraying and my mate to have acid splashed in his eyes... luckily he suffered no damage - we dunked his head in water so quickly. – Solar Mike Feb 22 at 22:07
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    @SolarMike my advice was to connect the frame point last, and disconnect it first, otherwise it's a useless precaution. How is that "bad advice"? What you have called "bad advice" was a remark that I have not had a problem. Just because you know it to have happened, does not make it a frequent danger. If it was, I would have been hurt by now. – Weather Vane Feb 22 at 22:14
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    @SolarMike I think the sentiment here is that we take these precautions certainly to avoid igniting hydrogen, but the actual instances of hydrogen exploding from a jump start are pretty uncommon. H2 is lighter than air and will self-ventilate if you give it the chance. Hydrogen can and does explode, but jumpstarting is relatively safe if you make sure and follow the correct precautions and procedures. – Christopher Hunter Feb 23 at 17:45
  • @ChristopherHunter thank you for your (expert) opinion - I will stick to my methods... – Solar Mike Feb 23 at 17:49
  • @SolarMike I don't see that you have mentioned any of your methods here. Would you be willing to share what they are and how they differ from other answers? – Christopher Hunter Feb 24 at 22:31
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They gave good advice except not the body ie a painted surface - use a big clean bolt or engine mount.

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Defeating the object ! Paint isn't a good electrical conductor - not even metallic paint !

The idea is that sometimes, when a battery has been charging, and therefore giving off explosive gases, any spark near is likely to cause a big bang. I did it as a teenager - only once, as it cost me a new battery and very nearly my health.

So the sensible thing to do is connect the positive to the battery terminal or lug, and the negative to any bare metal away from that - and connect that one last, disconnect first. Any subsequent spark would be away from the battery and its gases.

True, the metal bodywork on cars is part of the electrical circuit, but a chunky piece of metal, say on the engine, is going to make better contact. You might want to try connecting a clip from a jump lead to a painted surface: it just might work, if there was a bare patch there, but what a waste of time, energy and possibly paintwork.

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You can make either or both negative electrical connections to the frame instead of the battery.

When connecting, you must make the last (fourth) connection that way, because either battery can make hydrogen. When disconnecting, remove the first connection at a place away from the battery. Going from the 3rd to 4th connection is what causes the spark (connecting or disconnecting). With only 3 connections made, there is no circuit and no current.

Other than that, connection sequence isn't super important. All the guides that "train you by rote" into doing a very particular sequence, are simply trying to get you to follow those principles.

Making or breaking the circuit is what throws sparks. When only 3 ends are connected, the circuit isn't a circuit, and can't spark.

Clipping onto a painted surface won't work, and you shouldn't grab onto a visible place anyway, as you'll scratch the paint.

Look for relatively bare bolts under the hood, e.g. Macpherson strut tower, or on the lower frame under the bumper. In a pinch, lug nuts will do.

If need be, you can wiggle them to scrape off the rust and improve contact. Sparks when you do that are of no great concern, since they are away from the battery.

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  • Suggestion: Replace "You must MAKE the last connection that way," with "You must REMOVE the last connection that way,". Since the H2 potentially is a problem after it has been charged a bit and not on connection. – Hennes Feb 23 at 18:42
  • @Hennes I don't quite follow what you mean, but I cleaned it up. – Harper - Reinstate Monica Feb 23 at 19:41
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Or the positive terminal. It is typically the ultimate connection that makes the spark capable of igniting something.

Jump starting procedure:

  • Connect the positive terminal of the good car with the positive terminal of the bad car. which terminal you connect the jump lead clamp to first is relatively immaterial. I personally do the bad car first because if I drop the lead and accidentally short it, it's less of a risk on a battery with no charge on a stopped car than on a running car with a good battery.
  • Connect the negative terminal of the good car with a suitable point on the engine of the bad car. Connect the good car connection first. Connecting to the bad car engine ensures a good earth connection - the engine will have a few earth straps that connect to the body (and this to the battery). Some other answers have advocated connecting to a point on the suspension; I don't recommend this, chiefly because more of the suspension components are isolated from the body of the car by rubber bushes. The engine is a better bet, and make it some metal component that is directly bolted to the block of the engine, for example the power steering pump. Do NOT connect to metal pipes such as diesel injector pipes
  • Leave the good car engine idling, switch off accessories like headlights and heated windscreens. Leave the two cars connected for ten minutes

The idea isn't that the jumper cables can pass enough current to turn over the dead car (unless they are exceptionally well connected electrically, and very heavy gauge thickness of wire, and the dead car is a small petrol engine). The jumper cables exist to quickly supply a considerable charge to the dead battery, charging it up to a point where it can contribute to the power demands of starting the dead car. This is why you leave it for some time. Even the most weedy, rusty jumper cables you have kicking around the shed will be able to perform this task. The more dead the bad car is, the longer it'll need to be left. If it's been dead for months it may be impossible to successfully jump start because being in a state of extreme discharge kills lead acid batteries and if it cannot take a charge you're then relying on a jumper cable to pass the hundreds of amps needed; very unlikely to have a good enough connection for that

Many modern cars keep the battery in the trunk/rear of the car. If that's the case they usually have a jump start point under the hood, marked with a + so you don't have to pull the trunk carpet up

The bad car needs to have a good connection between its battery terminals and clamps and also between the battery negative and the car body. If you find that the bad car battery is fully charged but the car is behaving like it has a dead battery, a connection at the battery terminal to clamp or negative wire to car body (body earth wire) could be to blame

After you've waited, turn the bad car engine over (again turn off all lights and electrical accessories, press the clutch if it's a stick shift to reduce transmission drag) for a short period. Don't hold its starter motor active for a long time; it will quickly diminish the small amount of charge your leads have put into the battery and your leads will try and supply more and more power. This will be quite effective at heating them up, could set them on fire, and or damage the points where the clamps touch what they're clamped to

When the bad car is running, disconnect the end of the cable that is connected to the engine first

Take utmost care not to touch the positive jump lead clamp on any part of the car bodywork, especially if your jump leads are older types that don't have plastic insulation over most the positive clamp. Aside from risking damage to the electrical system of the car of a short occurs, you really do risk injury as the current that will flow is high enough to melt the metal where the contact is made (like a welder) and set plastic insulation on fire

Don't make the mistake that you can take the bad car for a quick drive to charge its battery. The best remedy for a dead battery is actually to trickle charge it, not jump it. Slamming lead acid batteries with charge buckles the plates causing internal damage to the battery and reducing its capacity to hold charge. If you can, put the bad car on trickle charge and leave it. If you have to use it urgently, jump it but finish off the battery charging process by trickle charging at home rather than driving it round for a long time - on some cars even if it's running the battery may be in too low a state of charge for the an intelligent charging system to start charging the battery. Trickle charging will always cost you (and the planet) less both in terms of not using fuel senselessly and also in terms of not risking ruining a battery that will then need replacement


If you need to replace your battery then you absolutely must remove the negative lead first and put it on last (unless you have a rare car that uses a body positive electrical system in which case you must remove positive first and put it on last)

The reason is one of safety. If you have both leads connected and you place a spanner on the positive clamp to undo it first, then EVERYTHING around that clamp is wired to the negative terminal of the battery. If you push the spanner too far, or it slips, and comes into contact with some part of the car while it is still connected to the positive clamp then a huge amount of current will flow through the spanner. Make no mistake; a car battery is capable of melting a spanner faster than you can let go of it

By disconnecting the negative lead first, if you catch the spanner on the car body while undoing the negative nothing happens. You will then have disconnected the negative (so the car body is no longer negative) so if you're then undoing the positive and catch the spanner on the car body, nothing happens

Refitting is the reverse; connect positive first, so if you catch the spanner on the body nothing happens (negative terminal of the battery isn't connected yet) and then connect negative- again so if you catch the spanner nothing happens (all the body is negative)

Remember, invert this advice only if your car is body positive. I know of no modern cars that are but it's a duty of responsibility to yourself to verify what your car is

Take care; batteries that flow hundreds of amps are not toys and are not low risk items

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